This invention relates to radio navigation receivers, and more particularly to a radio receiver adapted for use with the Global Positioning System (GPS) navigation system wherein a present position of the user station is determined based on respective ephemeris data received from a plurality of satellites.
In conventional GPS receivers, a present position of the user station is calculated based on ephemeris data obtained from the GPS satellites by receiving satellite signals as is well known in the art.
In the conventional GPS receiver of this type, to perform the positioning with updated ephemeris data of GPS satellites, if the power supply has been turned off, each ephemeris data obtained from the GPS satellites has been lost at all times.
The operation of such a conventional GPS receiver will be described with reference to a flowchart shown in FIG. 1, wherein ST1-ST4 designate operational steps to be performed by the GPS receiver.
Upon initiation of the GPS receiver, it is checked whether or not a respective GPS satellite is acquired by the GPS receiver (step ST1). If it is, respective ephemeris data is collected (step ST2).
It is then checked whether or not the collection of respective ephemeris data from the GPS satellites has finished (step ST3). If it has, the program goes to a position calculating routine wherein the position calculation of the user station is initiated based on the collected ephemeris data (step ST4), whereas if it has not, step ST4 is repeated.
Accordingly, the present position of the user station can be calculated based on the collected ephemeris data. However, in accordance with the conventional GPS receiver, it has been required to collect the ephemeris data once again even if the power has been turned off for a short period of time. This in turn results in a problem of delaying the initiation of position calculation, since it takes a considerable time for the GPS receiver to re-collect the usable ephemeris data.
It is therefore an object of this invention to eliminate the problem encountered in the conventional GPS receiver and to provide a GPS receiver capable of initiating position calculation with no delay based on the ephemeris data being stored when the power has been turned off, if the power is turned on again within a predetermined period of time after the power outrage.